I understand you have a questions regarding your hair and fear of losing it, but nobody really can give you straight answer.
It really depends on many factors and your mostly genetics. Are you experiencing shedding hair? Thinning? Your family history of losing hair? Mother, father side, brothers, sisters, uncles.
Let's say your hair started receding (let's call it mature hairline like everybody here does) It usually starts sometimes at 20 and develops until 30.
Some guys here started balding at 17, some lucky guys didn't develop any hair lose at 40. There are no rules really.
You can experience recession at your temples and It could stop after a while, It could stay like that for 25 years and then continue to progress.
You can take pictures every 3 months in same conditions (lightning, position, angle) and monitor it by yourself. Or you can look for some professional help but I really think you don't need it.
But I've said it before, If I were you I would pay more attention to my health and eating habits, because just like cars, if you tank them with shitty fluids it won't last as it should and you will experience a lot of problems. Human body isn't really any different.
